The long term objective of this work is to uncover guidance mechanisms that are critical to axon targeting during development, plasticity, and regeneration following injury. The discrete nature of taste papillae and the taste buds within their epithelium make it an excellent model for studying axon targeting mechanisms. Taste axons innervate taste buds but not adjacent epithelium, and somatosensory axons that penetrate non-taste epithelium do not enter taste bud epithelium. The majority of somatosensory axons terminate just beneath lingual epithelium and do not penetrate it. Surprisingly little is known about the mechanisms and molecules that underlie targeting in this system. Diffusible molecules such as Sema3A and neurotrophins have roles in repelling axons from and attracting axons to epithelial targets, respectively, but are unlikely to fully account for the ability of taste and somatosensory axons to distinguish between adjacent populations of target and non-target epithelial cells. Cell-attached cues could provide the signaling resolution needed for discrimination between adjacent target and non-target cell populations. Eph receptors (Ephs) and ephrins are cell attached cues that can mediate either stabilization (target recognition) or repulsion (non- target recognition). The first aim of the proposed work is to determine which Ephs and ephrins are expressed in gustatory epithelia in the tongue and palate and the sensory nerves that innervate them during embryonic development. Multiple approaches will be used to localize these proteins: immunohistochemistry, detection using fusion proteins containing the extracellular domain of their binding partners, detection of marker proteins substituted for Ephs or ephrins in mutant mice, and in situ hybridization. Second, to determine if Eph/ephrin signaling stabilizes or repels taste and somatosensory neurites, ganglion explants will be grown on substrates that have been micro-patterned with alternating stripes of Ephs or ephrins and control proteins. To determine if Ephs or ephrins are necessary for normal targeting, nerve trajectories in the tongues and palates of mutant mice lacking Ephs and/or ephrins will be evaluated. Identifying guidance molecules and understanding their roles in targeting to papillae and taste buds may uncover guidance mechanisms that apply to other parts of the nervous system in which the fidelity of initial innervation is not as obvious. These mechanisms may also be employed throughout life because peripheral epithelial targets routinely undergo turnover in the absence of injury and are also re- innervated after injury. PUBLIC HEALTH RELEVANCE: The goal of this work is to gain an understanding of the role of Eph receptors and ephrins in guiding axons to appropriate targets during development. This knowledge will aid not only in understanding how the nervous system develops normally, but will enhance our understanding of what factors may be defective in disease and also which factors can be manipulated to improve the prospects for recovery from injury or disease.